Hi Stuw
I don't think you're wrong at all. On my current build I've braced areas and I'll be adding some sound absorbing neoprene to help kill motor sounds. I started the thread here as its about 'R&D', so just exploring options really. How to stop the sounds in the first place rather than damp them once produced.
Some very interesting points raised I think, and I will be trying out some tests before committing into a build. A couple of observations having pondered on the issues raised:
1. While testing existing build on my desk, I had motors mounted on quite heavy aluminium angle plate, ESCs and Lipos, running no load at all. Significant noise from the motors, while they run very smoothly, the bearing noises are amplified when in contact with the desk. So this is not any alignment / vibration issue, pure motor sounds. I put a piece of neoprene under the mount, sound goes away. For me that confirms a rubber mount approach for the motors will yield results.
2. A conventional setup needs a bearing of some sort top and bottom of the prop tube, and usually a seal. But the P bracket approach with an exposed shaft, the P bracket could be considered to be the lower (shaft) bearing. And as Tim pointed out, there is a risk of the top bearing 'fighting' with the motors own bearings if alignment is not 100% perfect. So (assuming a solid coupling) why not consider the motors own bearings as the top 'tube' bearing, this can positively help with maintaining alignment. Then its just a case of using seals in the prop tube to keep the lake on the outside....
I think its well worth trying and will be doing so. And it may be useful during construction to just put plain bearings in to get the alignment right in the first place, then remove them and replace with seals once tube is bonded in. Then if it doesn't work, the plain bearings can always go back in... If it does work, the P bracket may be the only surface to surface contact of the whole drive train, and it could use a delrin or similar bearing / bush, water lubricated.
Or it may be garbage... We shall see....
Best regards
David.